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The organelle that fits both ideas is the mitochondrion. It’s the site of cellular respiration, housing the enzymes of the citric acid cycle and the electron transport chain. The inner membrane folds, called cristae, provide a large surface area where the electron transport chain creates a proton gradient that powers ATP synthase to make ATP. Mitochondria also contain their own circular DNA and ribosomes, reflecting their ancient bacterial origin and explaining why they can operate semi-independently from the rest of the cell. The nucleus stores DNA but isn’t where respiration occurs; chloroplasts have their own DNA and perform photosynthesis, not respiration; ribosomes lack DNA and are the protein-making machines of the cell.